Reference ID: MET-D1EE | Process Engineering Reference Sheets Calculation Guide
Introduction & Context
Extract purity is a key quality metric in process engineering that quantifies the fraction of the desired solute relative to the total dry mass of an extract. It is widely used in the food, beverage, pharmaceutical, and specialty-chemical industries to ensure that downstream unit operations (crystallisation, chromatography, spray-drying, etc.) receive feedstock of consistent potency. Typical applications include hop α-acids recovery, botanical extracts, and active pharmaceutical ingredients (APIs).
Methodology & Formulas
Define the mass of the target solute, \(m_{\text{target}}\), and the total dry extract mass, \(m_{\text{total}}\), both expressed in kilograms.
Compute the purity, \(P\), as a mass-ratio percentage:
\[
P = \frac{m_{\text{target}}}{m_{\text{total}}} \times 100\%
\]
To avoid division-by-zero, the denominator is replaced by \(\max(m_{\text{total}},\varepsilon)\) where \(\varepsilon\) is a small positive constant.
Parameter
Physical Constraint
Typical Range
\(m_{\text{total}}\)
\(m_{\text{total}} > 0\)
0.1–10 kg (lab to pilot scale)
\(m_{\text{target}}\)
\(m_{\text{target}} \geq 0\)
0–\(m_{\text{total}}\)
\(P\)
\(0 \leq P \leq 100\%\)
5–70 % (spectrophotometric assay calibration)
Process conditions such as temperature and pressure are recorded for traceability but do not enter the purity calculation itself.
For most solvent-based extractions, a mid-FT-NIR probe inserted directly in the raffinate line gives a 30–45 second loop time. If water content is the key impurity, a vibrating-tube density or dielectric constant sensor is simpler and updates every 5–10 s. Always validate the analyzer against lab GC or Karl-Fischer data during commissioning and re-check after any solvent change.
Take the sample from the last theoretical stage before the extract leaves the column, not from the pump-around return.
Install a mini knock-out pot with 5 min residence time to let entrained raffinate droplets settle; top layer is your representative extract.
If the column has side-draws, collect individual samples and build a purity profile; the lowest impurity value is your benchmark for control tuning.
Collect 50–100 lab data points under steady feed and solvent-to-feed ratio. Calculate the 3σ standard deviation of the impurity. Set the alarm at target + 3σ and the action limit at target + 4σ. Update limits every six months or after any catalyst or solvent change.
Check for response-factor drift—recalibrate the internal standard every shift for the first week.
Verify that the split ratio has not crept; even a 2 % change inflates light components.
Ensure the detector linearity range covers the highest component; area counts outside the linear zone are multiplied by an incorrect factor.
Worked Example – Extract Purity Assessment
A pilot-scale solvent-extraction unit is used to recover a high-value pharmaceutical intermediate. After phase separation and solvent stripping, 2.0 kg of wet cake is collected. Laboratory analysis shows that 1.8 kg of this material is the desired intermediate, with the remainder being residual solvent and trace impurities. Determine the extract purity at 25 °C and 1 bar.
Knowns
Total mass of wet cake, mtotal = 2.0 kg
Mass of target intermediate, mtarget = 1.8 kg
Temperature = 25.0 °C
Pressure = 1.0 bar
Step-by-Step Calculation
Extract purity is defined as the mass fraction of the target intermediate in the final product:
\[
\text{Purity} = \frac{m_{\text{target}}}{m_{\text{total}}} \times 100\%
\]
Insert the known masses:
\[
\text{Purity} = \frac{1.8\ \text{kg}}{2.0\ \text{kg}} \times 100\%
\]
Perform the division:
\[
\frac{1.8}{2.0} = 0.900
\]
Convert to percentage and round to three decimal places:
\[
0.900 \times 100\% = 90.0\%
\]
Final Answer
Extract purity = 90.0 % (mass/mass)
"Un projet n'est jamais trop grand s'il est bien conçu."— André Citroën
"La difficulté attire l'homme de caractère, car c'est en l'étreignant qu'il se réalise."— Charles de Gaulle
